tls: use CRYPTO_BUFFER to retrieve the peer certificate
tls: use CRYPTO_BUFFER to retrieve the peer certificate

Motivation:

BoringSSL uses CRYPTO_BUFFER to store the raw DER certificate already so we can make use of this and so allow to retrieve the DER certificate without the need to decode the X509 representation on each call.

Motiviation:

Use the CRYPTO_BUFFER via SSL_get0_peer_certificates

Result:

Less overhead when calling Connection::peer_cert()
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